Primary control circuit combined with secondary control circuit are widely applied in isolated switching mode power supplies. The control signals of the secondary control circuit is transmitted by a coupler to the primary control circuit to control the operation of the isolated switching mode power supply with the control signals of the primary control circuit. Generally, the secondary control circuit is powered by the output voltage of the isolated switching mode power supply.
In some conditions, for example, during the startup of the isolated switching mode power supply, or when the isolated switching mode power supply is shorted, the output voltage is not high enough to power the secondary control circuit. In that case, the control signal generated by the secondary control circuit may be wrong. As a result, the isolated switching mode power supply may work improperly.
The present invention pertains to provide an isolated switching mode power supply with high efficiency and the method thereof.